As the ability to fabricate greater numbers of transistors on a given semiconductor die size increases, the circuits on the die become more complex and require a greater number of external electrical connections. To accommodate the required number of external contacts, semiconductor packages have evolved from lead frame based packages, such as dual-in-line ("DIPs") and quad-flat-pack ("QFP") packages, to laminated substrate based packages, such as ball grid arrays ("BGA").
FIG. 1 is a cross-sectional view of a conventional BGA package. As shown, the package comprises a laminated substrate 106 formed of, for example, a ceramic or plastic material such as epoxy-glass. Electrically conductive traces (not shown) are formed on conductive layers of the substrate 106. Methods for forming conductive traces are known in the art. For example, photo-lithographic techniques may be used to image a desired pattern into a photo-resist material disposed on a conductive layer of the substrate 106. The photoresist material is then "developed," i.e., the photoresist material not exposed by the image, is removed, thereby creating a corresponding pattern of exposed conductive material on the substrate. The exposed conductive material is then removed in an etching process. Finally, the remaining photoresist material is removed, leaving the desired pattern of conductive material on the substrate.
Electrical connections between the layers are formed by conductive vias, such as via 114. Vias are formed in the substrate by known techniques, such as mechanical or laser drilling. After the via is created, it is plated with a conductive material to provide the desired electrical contact. A plurality of electrical contacts to the conductive traces of the package are provided on the lower surface of the package substrate 106. Solder balls 108 are attached to each of the contacts to allow electrical connection between the semiconductor package and external electronic components, such as printed wire boards. Solder balls 108 are conventional and typically are constructed from a lead-tin alloy and are attached to the contacts by well known methods such as re-flow soldering. Of course, other conventional electrical connectors could be substituted for the solder balls 108, such as conductive pins attached to contacts on the lower surface of substrate 106.
A semiconductor die 102 is mounted to the upper surface of the package substrate 106 by a suitable die attach material 110, such as epoxy. Electrical connection between bond pads on the die 102 and the conductive traces on the substrate 106 is provided by bond wires 104. Of course, in addition to the bond wires shown, different methods of providing electrical connections from the die to the package are also well known in the art, such as wire bonding and C4, or "flip-chip" techniques.
All semiconductor packages must not only provide for a number of electrical connections to be made from bond pads on a semiconductor die to external contacts on the package, but must also provide physical protection to prevent damage to the die during handling. To protect the die 102 and bond wires 104, a molded covering 112 is formed on the substrate 106. Generally, molded covering 112 is formed on the package by a transfer or injection molding process. Conventional transfer molding processes are known to those of skill in the art and are described in, for example, U.S. Pat. No. 5,635,671, issued to Freyman et al., incorporated herein by reference. This is explained in greater detail with respect to FIG. 2.
FIG. 2 is a cross-sectional view of a conventional two-piece mold. In this case, the mold comprises an upper mold section 200 which is adapted to mate with a lower mold section 202. The lower mold 202 has a recess formed therein for receiving and holding the semiconductor package during the molding process. The semiconductor package is conventional and includes a package substrate 106 having a semiconductor die 102 mounted thereon by means of a die attach material 110. Electrical connection between the die 102 and the substrate 106 is provided by bond wires 104. Lower mold section 202 also has an opening formed therein for receiving a transfer ram 210. The transfer ram 210 is slideably inserted into an opening in lower mold section 202 to force molding material into the cavity 204 as will be discussed in greater detail herein.
A cavity 204 is formed in the upper mold section 200 such that when the mold sections are placed together the cavity 204 is disposed over the die 102 and bond wires 104. As seen from the figure, the shape of the cavity 204 controls the shape of the molded covering. It will also be noted that the sides of the cavity 204 are provided with a slight slope to make removal of the package from the mold easier. Upper mold section 200 is also provided with a cavity 206 which is designed to receive the molding material 208. Cavity 206 is sometimes referred to as the "pot." Molding material 208 is generally a plastic material which hardens upon curing. A channel 212 is formed in upper mold section 200 which connects the pot 206 to the cavity 204. The channel 212 is sometimes referred to as the "runner" channel.
It will be understood by those of skill in the art that the mold sections are generally designed to fit within a frame, referred to as a "chase", during the molding process. The mold sections are often formed as separate removable units, referred to as "bars". The bar which corresponds to the upper mold section will be referred to herein as the "cavity bar". A multiplicity of mold cavities, such as cavity 204, are machined into the cavity bar to allow molding of several packages at once. The mold bars are typically removable from the chase so that the same chase can be used to mold packages of different sizes by simply substituting different mold bars. Similarly, a multiplicity of pots are also often formed on a separate bar, referred to as a "runner bar", to allow substitution of different pots for different packaging applications.
To facilitate simultaneous molding of a number of packages, individual substrates are provided on a substrate strip which contains multiple substrates which are later separated or "singulated", after the molding process. This is described in greater detail with respect to FIGS. 3A-3B.
FIG. 3A is a top view of a substrate strip 300. Substrate strip 300 contains four substrates, 302 (for simplicity, only two substrates are denoted by the number 302). Substrate strip 300 is also provided with a number of tooling holes, such as tooling hole 304, around its outer perimeter to allow the substrate strip 300 to be attached to other devices in the molding process, such as the cavity bars. Slots 306 are formed on substrate strip 300 between each of the substrates 302. Slots 306 operate to provide stress relief to the strip 300 to reduce or prevent warping during the molding process. Punching holes 305 are also provided on strip 300 at the comer of the substrates 302. Punching holes 305 are useful during the singulation process to reduce or prevent rough edges from being created on the substrates when they are punched out of the strip. Each substrate 302 is provided with a die attach area 307 to allow attachment of a semiconductor die (not shown) to the substrate 302. The die is attached both mechanically, by for example, die attach material, and electrically, by for example, bond wires or flip-chip solder ball connections.
It will be understood that substrates 302 are substantially the same as substrate 106 shown in FIG. 1 in that they are formed from a laminate material and are provided with conductive traces and vias, as discussed previously. The opposite side of strip 300 (not shown) is provided with conductive regions intended to provide electrical connection between conductive traces on the substrate and corresponding electrical contacts on external circuitry, such as printed wire boards ("PWB"). Such electrical connections are often referred to as second level interconnects and are implemented by, for example, solder balls as shown in FIG. 1.
FIG. 3B is a top view of conventional molding equipment with substrate strips attached. As shown, the molding equipment includes a mold chase 308. The mold chase allows attachments of cavity bars 310 on each side which are separated by runner bar 312. Substrate strips 300 are attached to cavity bars 310 by tooling holes 304 which fit over corresponding pins projecting from the top surface of the cavity bars 310. Slots 306 are, again, provided between the substrates 302 to help prevent warping of the strip 300.
Each substrate 302 is aligned with a cavity 204 (indicated by dotted lines) formed on the cavity bar 310. The strips 300 are disposed on the cavity bars 310 such that the upper, or die attach surface, faces the top surface of the cavity bar 310. Thus, the die and bond wires mounted to each of the substrates will be enclosed by the cavity 204. For simplicity, only one cavity is denoted by the numeral 204. It is to be understood, however, that the other cavities illustrated in the figure are substantially similar, and the following description is equally applicable to them.
Cavity 204 is connected by runner channel 212 to pot 206. Pot 206 is one of four substantially similar pots formed on runner bar 312. Each pot feeds two cavities on either side of the runner bar 312. However, this is for purposes of illustration, and numerous other arrangements are known in the art. Runner channel 212 joins cavity 204 at gate 316.
Since the molding operation is substantially the same for each of the substrate strips 300, it will be described with respect to a single package substrate 204 and its associated molding material pot 206, for the sake of simplicity. After the substrate strips 300 are attached to the cavity bars, the molding material is placed into the pot 206, and the bar containing the lower mold sections are pressed against the cavity and runner bars shown in the figure. This holds the substrate strips 300 firmly in place. Transfer rams (not shown) are then pushed into the pot 206 in order to force the mold material through runner channel 212 and gate 316 and into cavity 204. A sufficient amount of material is forced into cavity 204 to completely fill the cavity and enclose the die and bond wires (not shown) on the package substrate 302. After the mold material cures, the bars are separated and the substrate strips 300 removed. The package substrate 302 is then singulated by punching it out of the strip 300, resulting in a molded semiconductor die, such as that shown in the cross-section view of FIG. 1.
However, one shortcoming of the above process is apparent. Specifically, the mold material not only fills the cavity 204 but also the gate 316, runner channel 212 and pot 206. Therefore, when the mold material is cured, the cured material not only covers the die and bond wires, but also extends along the surface of the package substrate and outward from the package along the runner channel 212 to the pot 206. This excess cured material is often referred to as the "runner" and must be removed from the substrate strips before they are singulated.
Accordingly, gate 316 is generally made somewhat smaller in cross-sectional area than the runner in order to assist in the "degating" process, i.e., the removal of the excess material. Generally, the degating process is performed simply by peeling the runner away from the substrate until it separates at the gate. However, the mold material tends to adhere to the surface of the package substrate during the degating process. This adhesion is likely to cause damage to the surface of the package substrate as the mold material is peeled away due to tearing or rupturing of the substrate. The damage may be merely cosmetic, or if severe enough, may fracture the substrate or damage electrically conductive traces on the substrate surface, resulting in functional damage to the substrate. Additionally, peeling away the runner may also tear away part of the solder mask on the substrate surface, exposing the metal traces underneath to oxidation, or attack by other chemical pollutants. Even if no damage is done to the package substrate, the degating process may still weaken the seal between the molded body and the upper surface of the substrate, thus increasing the chances of moisture penetration in the package.
Efforts at addressing this problem involve providing a metallic trace, such as gold, on the upper surface of the package substrate which contacts the gate and runner. Since conventional molding materials do not adhere well to gold, it is easier, and less likely to cause damage, to peel away the runner. However, adding such an additional trace increases the overall cost of the package. Moreover, the trace also requires the use of a substantial amount of area on the package substrate, thus reducing the total amount of available area for routing conductive traces to be used for electrical connections on the package. Therefore, there is a need for a semiconductor package which allows easy removal of the runner without risk of damage to the package substrate. It is also desirable that such a package will not require additional manufacturing step, or a reduction in the amount of available area for routing conductive traces.